Exhaust Rattle

[shithappens]

My 2001 Silverado with a 5.3 has recently developed a buzzing type rattle somewhere in the exhaust system under the truck. It's not the heat shields as this has been looked at by a dealer twice.

 

Here are the symptoms,

 

* Only happens between 800-1100 rpm.

* Only happens when outside ambient temperature is 40 degrees or below.

* Only lasts until exhaust system warms up then diminishes almost completely.

 

I've had the dealer look into this for me twice with no results either time. They simply cannot find the "rattle", so I am hoping someone here may have come across this issue and can provide me a solution.

 

Also is there a possible TSB on this problem? If so, please let me know of it so I can direct the dealer to it.

 

Thanks again to everyone who can offer a helping hand.